A light-emitting diode (LED) is a semiconductor electronic device capable of emitting light and composed by p- and n-type semiconductor materials. It can generate self-radiating light in the ultraviolet light, visible light, and infrared light zones. Because an LED has the advantages of saving energy, long lifetime, and high brightness, in the trend of environmental protection, saving power, and reducing carbon emission, the applications of LED have become extensive increasingly, for example, traffic lights, streetlamps, flashlights, display devices, or lighting apparatuses.
Currently, most LED display devices, such as notebook computers or LCD panels, output pulse width modulation (PWM) signals as the light adjusting signals for LEDs using driving circuits. Thereby, the duty cycles of the switching signals can be adjusted and the LEDs are switched on and off in high frequencies for achieving the purpose of tuning their brightness. Please refer to FIG. 1, which shows waveforms of PWM and switching signals according to the prior art. As shown in the figure, when the PWM signal is high level, the switching signal S1 turns on the LED by high-frequency switching; when the PWM signal is low level, the switching signal S1 is lowered the low level to turn off the LED. Because the LED is turned off when the PWM signal is low level and turned on only when the PWM signal is high level according to this method, the glitter phenomenon, which is hardly aware by human eyes but harmful to visual perception, will occur.
Although the switching frequency of the LED by the switching signal S1 may be high (approximately between 170 and 270 Hz) and people cannot be conscious of the glitter phenomenon, the ciliary muscles in human eyes will respond to the glitter spontaneously. Therefore, staring at such display devices for a long time fatigues the ciliary muscles. Even worse, it can cause the problems of high intraocular pressure and feeling like vomiting.
Accordingly, the present invention provides a driving circuit for LED and a driving method thereof, which solve the glitter problem in LEDs by outputting a stable driving current to the LEDs.